Such devices which are known in the art comprise a detector for conversion of heat radiation into an electrical signal, an optical system which images the heat radiation emanating from an object of measurement onto the detector, a heat-conducting temperature equalization element which keeps the detector and the optical system at a common temperature, a temperature sensor which measures the temperature of the temperature equalization element, as well as a housing which encloses all the components.
A detector which is very frequently used is the so-called thermopile. This consists of a row of pairs of thermo-elements disposed in a circle, the hot solder joints forming an inner circle and the cold solder joints forming an outer circle. The inner circle with the hot solder joints is typically located on a thin film, whilst the outer circle with the cold solder joints is connected to the temperature equalization element. The hot solder joints are blackened in order to increase the degree of absorption. The heat radiation (IR radiation) striking the hot solder joints from the optical system heats the hot solder joints to a temperature different from the cold solder joints. Due to the Seebeck effect a voltage is produced from which the temperature of the object of measurement can be determined.
In order to protect the thermopile from environmental influences and in particular to keep the time constant and the sensitivity of the thermopile independent of the composition of the air and humidity, the thermopile is hermetically enclosed. The detector housing typically consists of a metal transistor housing with an IR window. It is filled with a defined gas, usually an inert gas such as argon or neon. However, these devices, which are also known by the term pyrometers, have the disadvantage that when measurements are carried out below 0.degree. C. and at high air humidity moisture can penetrate into the device. In the optical system bedewing with water or freezing out of ice can occur, rendering temperature measurement impossible, since water and ice are impermeable to IR rays.
The object of the invention, therefore, is to make further developments so that temperature measurements are also reliably possible below 0.degree. C.